1. Field of the Invention
The invention relates to a method of and apparatus for ascertaining the cylinder pressure in an internal combustion engine during operation of the engine.
2. Description of the Prior Art
Modern combustion processes for instance homogeneous or partly homogeneous combustion processes, of the kind known in diesel engines require the most accurate possible knowledge of the course of the pressure in the work chamber of the applicable cylinder. The cylinder pressure is determined by parameters that individually describe the characteristics of the combustion process. Often, these combustion processes prove very vulnerable to altered conditions, such as conditions in the injection quantity or the air mass or the proportion of the recirculated exhaust gas. To achieve the required stability of the combustion process, pure open-loop control itself is therefore inadequate. Instead, stable operation of the combustion processes in question must be attained by closed-loop control or regulation, in closed control loops. This is possible with the aid of suitable information about the course of combustion.
Independently of the regulation of modern combustion processes, abnormal combustion processes (such as knocking in Otto engines, or misfires) can also be detected by ascertaining the pressure course. With the aid of the ascertained cylinder pressure, further fundamental conclusions for regulating or controlling the engine and diagnosing it can also be drawn. Consequently, the pressure course is an important basic variable.
For pressure measurement, it is known to use special invasive pressure sensors, which are either combined with spark plugs or glow plugs or are additionally built into the engine. This involves direct ascertainment of the course of the cylinder pressure. The disadvantages are the comparatively great effort and high costs involved. The sensors are additionally exposed to severe stresses in the combustion chamber.
Alternative measuring methods for ascertaining the course of the cylinder pressure make use of special ring sensors, which are screwed into the spark plug conduit (see European Patent Disclosure EP 0621470) or which propose the use of capacitive travel sensors on engine components (see German Patent Disclosure DE 19803470). While the method of EP 0621470 involves a direct pressure measurement method, in DE 19803470 the cylinder pressure is ascertained indirectly (via the changes in capacitance of the travel sensors).
The disadvantages of the two above-mentioned known measuring methods is that one sensor per cylinder is needed; a signal with further influences superimposed on it must be expected (the method of DE 19803470), the sensitivity is poor, or among other factors maintenance is made more difficult.
In an indirect measuring method, which has become known from German Patent Disclosure DE 10327147 A1, for ascertaining the course of the cylinder pressure the expansion of the outer cylinder wall is supposed to be measured with the aid of strain gauges. A disadvantage of this method is that the cooling conduits that often extend between the inner and outer cylinder walls lessen the measurement effect, and for each cylinder to be sensed, a separate sensor of its own must be used. Measuring the expansion at the outer cylinder wall moreover requires that the suitable positions at the engine be available and not occupied by attached parts, or that the expansion field is unimpeded by receptacles, cast integrally onto the engine, for additional devices. These prerequisites are met only rarely in practice, given the tight spatial conditions as a rule in the engine compartment of motor vehicles. Moreover—especially in view of new engine constructions—it is in no way certain that a fundamentally sufficient expansion amplitude will be available on the sidewall of the engine. Basically, applying sensors for strain measurement on the sidewall of the engine is complicated and as a rule requires modification both of the cast engine block and of its machine. Sensors disposed there are necessarily exposed to environmental factors. A primary example that can be named here is corrosion of the interface with the engine that is decisive for transmitting the signal and consequently for the sensor output signal.
From German Patent Disclosure DE 10108051, it is also known to indirectly ascertain the course of the cylinder pressure by measurements of expansions and changes in travel of the cylinder pressure screws. A disadvantage here is the limited installation space in the cylinder head as well as the major thermal stress. In addition, it must be expected that because of the immediate vicinity of strong sources of structure-borne sound (valve operation, impacts of the valve plate, valve motion, interference from injectors), interference in the output signal will be caused.
Finally, from German Patent DE 3933947 C1, an indirect measurement method for determining the combustion pressure in reciprocating piston engines has become known which is based on the provision of measuring accelerations at each crankshaft bearing of the engine and from that drawing a conclusion about the course of the combustion pressure. For this purpose, piezoelectric acceleration sensors mounted on the crankshaft are said to be used. The disadvantage is that the acceleration values furnished by the sensors must first be converted into the force conditions that actually pertain.